Silicoaluminophosphate (SAPO) molecular sieves contain a three-dimensional microporous crystal framework structure of [SiO2], [AlO2] and [PO2] corner sharing tetrahedral units. The [PO2] tetrahedral units are provided by a variety of compositions including phosphoric acid, organic phosphates such as triethyl phosphate, and aluminophosphates. The [AlO2] tetrahedral units are provided by a variety of compositions including aluminum alkoxides such as aluminum isopropoxide, aluminum phosphates, aluminum hydroxide, sodium aluminate, and pseudoboehmite. The [SiO2] tetrahedral units are provided by a variety of compositions including silica sols and silicon alkoxides such as tetraethylorthosilicate and fumed silica.
Aluminophosphate (ALPO) molecular sieves are crystalline microporous oxides which can have an AlPO4 framework. ALPOs can have additional elements within the framework, typically have uniform pore dimensions ranging from about 3 Angstroms to about 10 Angstroms.
Light olefins, particularly ethylene and propylene, can be produced by contacting a feedstock containing oxygenated organic compounds, such as methanol or dimethyl ether, with a SAPO and/or ALPO-containing catalyst. See for example U.S. Pat. No. 4,499,327.
SAPO-34 and SAPO-18 have been reported as suitable catalysts for light olefin production from methanol. SAPO-34 belongs to the family of molecular sieves having the structure type of the zeolitic mineral chabazite (CHA). The preparation and characterization of SAPO-34 has been reported in several publications, including U.S. Pat. No. 4,440,871; J. Chen et al. in “Studies in Surface Science and Catalysis”, Vol. 84, pp. 1731-1738; U.S. Pat. No. 5,279,810; J. Chen et al. in “Journal of Physical Chemistry”, Vol. 98, pp. 10216-10224 (1994); J. Chen et al. in “Catalysis Letters”, Vol. 28, pp. 241-248 (1994); A. M. Prakash et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 90(15), pp. 2291-2296 (1994); Yan Xu et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 86(2), pp. 425-429 (1990), all of which are herein fully incorporated by reference.
SAPO-18 belongs to the family of molecular sieves having the AEI structure type. Other molecular sieves with the AEI structure type are ALPO-18 and RUW-18. The preparation and characterization of molecular sieves with AEI structure type has been reported in several publications, including U.S. Pat. No. 4,440,871; J. Chen et al. in “Studies in Surface Science and Catalysis”, Vol. 84, pp. 1731-1738; U.S. Pat. No. 5,279,810; J. Chen et al. in “Journal of Physical Chemistry”, Vol. 98, pp. 10216-10224 (1994); J. Chen et al. in “Catalysis Letters”, Vol. 28, pp. 241-248 (1994); A. M. Prakash et al. in “Journal of the Chemical Society, Faraday Transaction” Vol. 90 (15), pp. 2291-2296 (1994); Yan Xu et al. in “Journal of the Chemical Society, Faraday Transactions” Vol. 86(2), pp. 425-429 (1990); U.S. Pat. No. 5,609,843, all of which are herein fully incorporated by reference.
International Application PCT/NO97/00272, published as WO 98/15496, relates to a molecular sieve that is a mixed phase comprising silicoaluminophosphates of AEI and CHA structure, referred to as RUW-19. RUW-19 is said to produce an X-ray diffractogram that includes peaks at the following 2θ values: 9.3-9.5, 10.4-10.6, 12.7-12.9, 13.8-14.0, 15.9-16.1, 16.7-16.9, 18.9-19.0, 20.5-10.7, 21.0-21.3, 23.7-24.0, 25.7-26.0, 30.9-31.1. FIG. 1 of this publication shows the XRD-traces in the 15-33 (2θ) region for RUW-19 (examples 1 to 3), SAPO-18 (example 4), SAPO-34 (example 5) and a physical mixture of SAPO-18 and SAPO-34 (example 6). This publication suggests that RUW-19 is different from a physical mixture of SAPO-18 and SAPO-34. RUW-19 has peaks characteristic of an AEI structure type molecular sieve, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in SAPO-18 or AEI. Also, RUW-19 does not have the reflections associated with SAPO-34 or CHA centered at 17.8 (2θ) and 24.8 (2θ). According to WO 98/15496, RUW-19 is suitable for the conversion of methanol to olefins. Throughout this description, the XRD reflection values are referred to as (2θ), which is synonymous to the expression “degrees 2θ”.
In the conversion of methanol to olefins, SAPO-34 exhibits relatively high product selectivity to ethylene and propylene, and low product selectivity to paraffin and olefin with four or more carbons (C4+ olefin). Catalysts containing SAPO-34 are thus particularly suited for the conversion of methanol to olefin. Despite its good performances, carbonaceous deposits, commonly referred to as coke, quickly form in the catalytic cages of SAPO-34. Eventually, the presence of too much coke will clog up the cage and deactivate the catalyst. Also, despite its low product selectivity to paraffin, SAPO-34 still produces by-products. Separating by-products from the desired ethylene and propylene adds additional cost to the methanol to olefin conversion process. Therefore, there is a need to find new molecular sieves that have good product selectivity and produce few by-products.